This invention relates to bonded electrical connections for electrical components. Specifically, this invention is an improved connection for small components having metallic electrodes or wires by which they are connected to circuit boards or other conductors.
Connecting an electronic device to a circuit board or substrate is usually accomplished by soldering, a process wherein an electronic device having a metal wire lead attached to it is heated together with a circuit board or substrate and to both of which is applied molten solder, typically a mixture of tin and lead. The molten metal solidifies around the circuit board and lead, electrically and mechanically bonding the circuit board and lead together.
Other methods of connecting components together use electrically conductive adhesives which are useable over a range of temperatures. Some of these adhesives include electrically conductive organic based adhesives and inorganic based adhesives.
A principle objective in connecting electronic devices together is the maintenance of the electrical connection despite mechanical stresses imposed on the joint. The mechanical strength of an electrical connection is dependent upon not only the type of bonding agent used to join the conductors but also upon the geometry of the conductors being joined.
A simple butt joint, which is a joint comprised of a wire held nearly perpendicular to a flat circuit board, holds the wire in place principally by virtue of shear forces in the bonding agent. It is well known that solder joints and adhesive joints that rely on shear strength alone are the weakest type of joints. To improve the strength of connections made with solder or adhesives, different lead geometries which offer the ability to resist force in shear, compression, and tension are frequently used.
One lead shape which improves the strength of a joint is a gull-wing lead (so named because of the shape of the lead after it is bent) which increases the area of the lead tangent to the surface of a circuit board, thus increasing resistance to tensile forces. The gull-wing joint requires a longer lead length to permit bending the lead so that the lead is not joined at right angles to the circuit board. The gull-wing also requires a larger footprint on a circuit board because of its shape.
Another lead shape which improves the strength of a joint is a J-lead, so named because the lead of a component is bent under the component to resemble the letter "J". The J-lead, like the gull-wing, requires more lead length, but for a given connection requires less circuit board area than the gull-wing connection. The principle drawbacks of the J-lead are its cost and the difficulty associated with inspecting the completed connection. Since the J-lead is bent under a component, it is difficult to inspect after the soldering process.
The butt joint, gull-wing and J-lead connections also suffer from solder wicking, a phenomenon in which molten solder migrates along the lead. Solder wicking produces localized collection of solder at bends in the leads which reduces the ability of the lead to flex. This reduced compliance of the leads increases failures of solder joints. In addition to solder wicking, gull-wing and J-lead connections are also difficult to manufacture with uniform lengths because of the plastic and elastic deformation of the bent leads. Components using gull-wing and J-lead leads, after bending, are frequently of slightly different heights. The ends of the leads are not coplanar. This lack of coplanarity makes gull-wing and J-leads difficult to use because one slightly longer lead electrically disconnects shorter leads from a circuit board.
An electrical connection useable with solder or adhesives, which is easy to fabricate, minimizes circuit board space usage, improves the strength of a joint over simple butt joints, has coplanar leads and eliminates solder wicking would be an improvement over the prior art.